2) q = 1/3 b5 = 1/81

b5 = b1*q^4

b1 = b5/q^4 = (1/81) * 81 = 1

Sбеск убывающей геом прогрессии = b1/1-q = 1/(1 - 1/3) = 1/(2/3) = 1.5

4) q = -1/2

b4 = 1/8

b4 = b1*q^3

b1 = b4/q^3 = 1/8 * (-8) = -1

S = b1/1-q = -1 / (1 + 1/2) = -1/(3/2) = -2/3